It is well known that the rebars (short for reinforcing bars) in a steel reinforced concrete structure may suffer from corrosion. In particular in environments where sea water or deicing salts are present, chlorides may penetrate into the structure during a period of several decades unnoticed, until the chloride content at the rebar surface inside the structure reaches a critical level and initiates corrosion.
At the surface of the rebars embedded in the concrete, corrosion may result in the formation of voluminous corrosion products (rust) which have two to four times the volume of the original steel and no desirable mechanical properties. The corrosion products may cause the formation of cracks in the embedding concrete, which may eventually spall off at the surface. Corrosion may also produce (corrosion) pits or holes in the surface of the reinforcing steel, thereby reducing the strength of the structure due to a reduced cross-sectional area of the rebars.
Conventional methods of repairing a corrosion affected structure aim to replace chloride contaminated concrete in the structure with chloride-free concrete. This is typically done by removing cracked and spalled concrete and breaking out additional concrete until the depth of chloride penetration, thereby exposing the rebars; cleaning the surface of the rebars, for instance by grit blasting, and, eventually, applying new and chloride-free concrete. Research has shown, however, that about half of such conventional repairs fail within ten years, which is considered too short with regard to the usually required duration of 20-50 years and the cost of the repairs. See for more information Tilly, G. P., Jacobs, J., 1007, Concrete repairs—performance in service and current practice, IHS BRE Press, Bracknell, ISBN: 978-1-86081-974-2. Conventional repair failure has been found to be often attributable to one or more of the following causes: (i) insufficient concrete removal, leaving some chloride contaminated concrete in place; (ii) insufficient cleaning of the affected reinforcing steel, leaving corrosion products and chloride ions in the corrosion pits; and (iii) electrochemical effects between repaired and surrounding non-repaired locations where chlorides are present.
As alternatives to conventional methods of repair, various electrochemical methods have been proposed in the art. One such method is chloride extraction, in which chlorides are caused to migrate into an external electrolyte under the influence of an electric field. The chlorides accumulate in the electrolyte, and are eventually discarded together with the electrolyte, such that the concrete structure is left in a state without chlorides in which the rebars may repassivate. A notorious drawback of chloride extraction is the unpredictability of its duration. A treatment may last anywhere between several weeks and several months, and during this period samples of the concrete must be taken and analyzed to determine residual chloride levels to monitor how the treatment advances.
U.S. Pat. No. 6,322,691-B1 (Miller) has suggested another exclusively electrochemical curative treatment for chloride-induced corrosion affected steel reinforced structures. The treatment entails establishing a distributed direct current between the reinforcing steel, connected as a cathode, and an external, distributed electrode, connected as an anode. The current may have a density of at least 0.1 Ampere per square meter of surface area of the reinforcing steel, and be passed for a time sufficient to provide a total charge of at least about 100, but not substantially more than 2,000, Ampere-hours per square meters of surface area of the reinforcing steel. The distributed direct current is contended to cathodically strip the rebars, in the sense that any existing oxide or other films on their surface are removed. At the same time, the rebars are negatively charged, which causes chloride ions to be strongly repelled from the steel surface and driven back into the surrounding concrete. This would render the surrounding concrete essentially chloride free to a distance of usually at least 10 mm from the steel. Accordingly, once the electrochemical treatment is ceased, the rebars, which are now in a clean, active and chloride-free environment, are alleged to be given the opportunity to slowly repassivate by forming a dense protective oxide film (also known as a ‘passivating film’) to protect the steel from corrosion. The corrosion protection imparted this way is suggested to be long lived, and robust against new penetration by chloride ions.